Disorders that result from improper lung development are often fatal. The nonfatal disorders frequently result in long-term disabilities, and are associated with enormous healthcare expenditures. Although epithelial-endothelial interactions are thought to mediate lung development, the precise molecular and cellular mechanisms of these interactions are not well understood. The goal of this project is to define the role of endothelial Notch signaling in epithelial lung development Notch signaling is critical for early vascular development and Notch receptors and ligands are found in large numbers in the developing lung, predominately in the endothelium. When Notch signaling is blocked in lung bud culture, lungs undergo ectopic budding and increased branching morphogenesis. Based on the increased branching of In vitro Notch-inhibited lungs and Notch's known role as an inhibitor of sprouting angiogenesis, we hypothesize that there is an inverse relationship between endothelial Notch signaling and lung epithelial branching morphogenesis that is mediated bv a paracrine mechanism. In aim 1, we will express a constitutively-active form of Notch4 specifically in the endothelium under tetracycline regulation for variable intervals during lung development in mouse embryos. Using CD31 and E-cadherin immunohistochemistry, we will determine whether there is a decrease in branching and whether that decrease corresponds to a decrease in capillary density. In aim 2. we will delete the canonical downstream Notch transcription factor RBP-JK in the endothelium for variable intervals during lung development in mouse embryos. Embryonic lungs will be analyzed in the same way as in aim 1, but in aim 2, we expect to see an increase in branching that corresponds to an increase in capillary density. Lastly, in aim 3, we will grow lung buds from our transgenic embryos in culture to determine if the relationship between Notch signaling and branching persists in the absence of circulation. To test for a paracrine mechanism, we will extract endothelial cells from the lungs of our transgenic embryos and co-culture them with lung epithelial cells that are physically separated by a transwell filter. We expect the proliferation and migration of these epithelial cells to decrease with endothelial Notch loss-of-function and increase with Notch gain-of-function. Relevance: Disorders of lung development lead to deadly and costly diseases in neonates and children. The first step to combat these diseases is to understand the normal process of lung development. This project seeks to define the relationship between signaling in growing lung blood vessels and the associated coordinated development of airways.